Summary Our goal in this project is to develop a new class of electrical recording device that complements and piggy- backs on cutting edge imaging technologies. Whereas multi-electrical recording has provided detailed measurements of neural activity with high temporal precision, it is also invasive, provides relatively low spatial resolution, and provides little information about the identity of measured neurons. Optical imaging techniques, conversely, provide very fine spatial resolution, easing neural identification, but at the cost of significantly worse temporal resolution, and with the requirement of either chemical (through fluorescent dyes) or genetic modification of the tissue. In order to better bridge these two modalities, we envision developing untethered Microscale Optoelectronically Transduced Electrodes (MOTEs) which combine optoelectronic elements for power and communication with custom CMOS circuits for low-noise amplification and encoding of electrical signals. Each MOTE will be powered by optically stimulated micro-photovoltaic cells and will use the resulting 1-2W of electrical power to measure, amplify, and encode electrophysiological signals, up-linking this information optically by driving an LED. MOTEs will avoid many of the problems associated with standard wire- and shank-based electrodes, where most of the volume of the implanted electrode, and so most of the tissue damage it does, stems from the long rigid shank that connects electrode sites to external electronics. To be most useful, MOTEs' photovoltaics will be designed to harvest power from optical stimuli of the same wavelengths and intensities as are used in stimulating fluorescence when imaging neural activity. Similarly, the LED used for uplink will be designed to emit light at wavelengths and intensities consistent with those detectable by a fluorescent imaging system. These choices will allow the both down- and up-link of optical signals to be handled by existing imaging systems with minimal modification. By employing a pulsed stimulation (as is used in multi-photon systems) and appropriately encoding and timing up-linked LED pulses, fluorescent and MOTE emissions can be segregated into adjacent sub-microsecond time bins. This combination of optical compatibility and temporal multiplexing will allow simultaneous imaging and electrical recording of neural activity from the same volume of neural tissue, using the same optical imaging and recording systems. This simultaneous, heterogeneous measurement capability will enable a much wider range of experiments and studies of neural activity than are presently possible.